Nitride semiconductor of elements in Group III or V in the periodic table occupies an important position in the field of electronic and opto-electronic devices, and this field will be more important in the future. In practice, nitride semiconductor has a wide range of applications, ranging from laser diodes (LDs) to transistors that can operate at high frequency and high temperature. Furthermore, the range of applications includes ultraviolet light detectors, elastic surface wave (SAW) devices and light emitting diodes (LEDs).
For example, gallium nitride is known as a suitable material for applications of blue LEDs or high temperature transistors, but is being widely studied for the use of microwave electronic devices but not limited thereto. Furthermore, as stated herein, gallium nitride has a wide range of applications including gallium nitride-based alloys such as aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN) and aluminum indium gallium nitride (AlInGaN).
In devices using nitride semiconductor such as gallium nitride, the most frequently used substrate for growing a nitride semiconductor layer is a “heterogeneous” substrate such as sapphire, silicon carbide (SiC), and silicon. However, because these heterogeneous substrate materials have a lattice constant mismatch and a difference in thermal expansion coefficient with nitride, a nitride semiconductor layer grown on the heterogeneous substrate includes many crystal defects such as dislocation. The defect acts as a key factor that degrades the performance of nitride semiconductor devices such as LEDs.
Because a sapphire substrate has a higher thermal expansion coefficient than gallium nitride, when gallium nitride is grown at high temperature and then cooled down, compressive stress is applied to a gallium nitride epitaxial layer. Because a silicon substrate has a lower thermal expansion coefficient than gallium nitride, when gallium nitride is grown at high temperature and then cooled down, tensile stress is applied to a gallium nitride epitaxial layer. For this reason, a substrate warpage phenomenon occurs, and to prevent substrate warpage, the substrate thickness increases. The use of a thick substrate only contributes to the reduced superficial phenomenon, and is not technology that reduces the stress of a thin film itself. If the stress of a thin film itself is reduced, an advantage is that a thin substrate can be used. Furthermore, to separate a chip from a fabricated LED, a substrate needs to be ground while leaving about 100 μm, and in this circumstance, if a thin substrate can be used, great benefits will be gained in the aspect of LED production.
In some cases, the nitride semiconductor layer formed on the heterogeneous substrate should be separated from the heterogeneous substrate according to the need, and prior art proposed laser lift-off. However, even though a laser lift-off process is used, substrate warpage occurs due to a difference in thermal expansion coefficient between a sapphire substrate and nitride semiconductor, or a nitride semiconductor layer is melted using a laser and peeled off, so a side effect is the occurrence of thermal stress during the process due to high temperature heat at a local area. The laser lift-off process involves thermal and mechanical deformation and decomposition of nitride semiconductor. Due to laser beam impacts, the nitride semiconductor layer is susceptible to defects such as cracks and the nitride semiconductor layer may be damaged, and further, the nitride semiconductor layer is prone to breakage, and thus, the process is unstable.
Therefore, there is the demand for a substrate separation method with high reliability or a method for obtaining a high quality nitride semiconductor substrate or nitride semiconductor device.